Very odd.
This almost certainly has to be specific to that user so your support of it is going to be quite limited because you can't necessarily fix an issue that only affects one person...
But there may be some clues to be had.
First of all, if you get him to View Source on an affected page, does he see these two lines:
<link rel="stylesheet" href="css.php?css=xenforo,form,public&style=1&dir=LTR&d=1340105131" />
<link rel="stylesheet" href="css.php?css=attachment_editor,bb_code,editor_ui,message,message_user_info,quick_reply,share_page,thread_view,xfcom_page_wrapper&style=1&dir=LTR&d=1340105131" />
AND Importantly, does he see any other CSS in there? Maybe he has an add-on which injects custom CSS into a page. Would be useful if he tried your site with all browser add-ons disabled.
These are where css.php loads all of the CSS templates.
If he does see them, it may be worth him trying to physically go to those paths, e.g.
http://www.promotion-box.com/css.php?css=xenforo,form,public&style=7&dir=LTR&d=1345378635
This should contain default XenForo css.
Also:
http://www.promotion-box.com/css.ph...view&style=7&dir=LTR&d=1345378635
That will contain all the other CSS for your site.
What output does he get? In theory he should be seeing your compiled CSS. Maybe he's getting an error when he access them which will help identify the issue.
Also, in Chrome, are there any errors in the Console? (F12 > Developer Tools > Console).
